During the past years, the interest in using mobile and landline/wireline computing devices in day-to-day communications has increased. Desktop computers, workstations, and other wireline computers currently allow users to communicate, for example, via e-mail, video conferencing, and instant messaging (IM). Mobile devices, for example, mobile telephones, handheld computers, personal digital assistants (PDAs), etc., also allow users to communicate via e-mail, video conferencing, IM, and the like. Mobile telephones have conventionally served as voice communication devices, but through technological advancements they have recently proved to be effective devices for communicating data, graphics, etc. Wireless and landline technologies continue to merge into a more unified communication system, as user demand for seamless communications across different platforms increases, which in turn creates more usage, and leads to more services and system improvements. Additionally, these communication systems can evolve into new generations or partial generations of communication systems.
For example, one such evolved network is based upon the Universal Mobile Telephone System (UMTS) which is an existing third generation (3G) radiocommunication system that is evolving into High Speed Packet Access (HSPA) technology. Yet another example is the introduction of another air interface technology within the UMTS framework, e.g., the so-called Long Term Evolution (LTE) technology.
Regarding the air interface for high-speed wireless communications, multiple input multiple output (MIMO) antennas are often used. MIMO can provide both diversity gain and multiplexing gain. MIMO can enable the simultaneous transmission of multiple streams of data with each stream being called a layer. The number of transmit antennas, receive antennas, and layers can be denoted by NT, NR, and R, respectively, with R never being greater than NT, and NR usually being greater than or equal to NT for the uplink (but not necessarily the downlink).
MIMO typically assumes the use of a precoder, which is mathematically expressed as a left multiplication of a layer signal vector (R×1) by a precoding matrix (NT×R), which can be chosen from a pre-defined set of matrices. These pre-defined sets of matrices are also known as so-called codebooks, examples of which can be seen in FIGS. 1-4 with FIG. 1 showing a codebook 2 for a two transceiver system and FIGS. 2-4 showing a codebook 4 for a four transceiver system. (Note that the r-th column vector of the precoding matrix represents the antenna spreading weight of the r-th layer.)
The precoding matrix usually includes linearly independent columns, and thus R is referred to as the rank of the codebook. One aspect of using this kind of precoder for transmitting signals is to match the precoding matrix with the channel state information (CSI) so as to increase the received signal power and also, to some extent, reduce inter-layer interference, thereby improving the signal-to-interference-plus-noise ratio (SINR) of each layer. Consequently, the precoder selection is improved when the transmitter knows the channel properties. In general, the more accurate the CSI that the transmitter obtains, the better the precoder match.
LTE uses orthogonal frequency division multiplexing (OFDM) in the downlink and discrete Fourier transform (DFT)-spread OFDM in the uplink. Sounding reference signals (SRS) are used by the base station to obtain information about the uplink channel, i.e., in the transmit direction from a mobile station or UE toward a NodeB. The basic principle behind sounding is that the UE periodically transmits a wideband signal according to a configuration sent from the NodeB to the UE. Since the SRS is known by the NodeB it can be used to calculate a CSI for the UE's uplink channel which in turn can be used by various RRM-related algorithms such as scheduling, link adaptation and power control. However, the CSI obtained from SRS can, in some cases, deviate from the radio channel more than is desirable.
Accordingly, it would be desirable to have other methods and systems to obtain the CSI for use in, for example, precoder selection.